The present invention relates to a low-pass filter and specifically to a technique of a low-pass filter suitable for use as a loop filter in a feedback system, such as a phase locked loop circuit, a delay locked loop, or the like.
In currently-existing semiconductor integrated circuit systems, a feedback system, especially a phase locked loop circuit (hereinafter, referred to as “PLL”), is one of the indispensable components and is incorporated in almost all the LSI devices. The applications of the feedback system range over various technological fields, such as communication devices, microprocessors, IC cards, etc.
FIG. 13 shows the structure of a general charge pump type PLL. General features of the PLL are described with reference to FIG. 13. A phase comparator 10 compares input clock CKin which is supplied to the PLL and feed back clock CKdiv and outputs up signal UP and down signal DN according to the phase difference between the compared clocks. A charge pump circuit 20 outputs (releases or sucks) electric current Ip based on up signal UP and down signal DN. A loop filter 30 smoothes electric current Ip and outputs voltage Vout as a result of the smoothing of electric current Ip. A voltage controlled oscillator 40 changes the frequency of output clock CKout of the PLL based on voltage Vout. A frequency divider 50 divides output clock CKout by N, and a resultant clock is fed back as feedback clock CKdiv to the phase comparator 10. By repeating the above operation, output clock CKout gradually converges on a predetermined frequency and is locked.
The loop filter 30 is an especially significant component among the above components of the PLL. It can be said that the response characteristic of the PLL is determined according to the filter characteristics of the loop filter 30.
FIGS. 14A and 14B show general loop filters. FIG. 14A shows a passive filter. FIG. 14B shows an active filter. These filters are equivalently replaceable with each other and have the same transfer characteristic. As seen from FIGS. 14A and 14B, the loop filter 30 is substantially a low-pass filter formed by a combination of a resistive element and a capacitive element irrespective of whether it is a passive filter or an active filter.
According to the control theory for PLLs, the response bandwidth of the PLL is preferably about a 1/10 of the frequency of the input clock at the maximum. If this theory is followed, in a PLL which receives a reference clock having a relatively low frequency, it is necessary to reduce the cutoff frequency of the loop filter such that the response bandwidth is narrowed. Thus, a loop filter in a conventional PLL has a relatively large time constant, i.e., a large CR product. In general, a larger capacitive element is used in order to achieve a larger CR product.
However, increasing the size of the capacitive element is causes an increase in the circuit size. This is a serious problem especially in a semiconductor integrated circuit including a large number of PLLs, such as a microprocessor, or the like. Further, especially in an IC card, it should be avoided, in view of reliability, to incorporate an element thicker than the card. The countermeasure of externally providing a large capacitive element is substantially impossible. Conventionally, the following means have been provided for the purpose of decreasing the size of the capacitive element of the loop filter.
In the first countermeasure example, a loop filter is structured such that a capacitive element and a resistive element, which would generally be connected in series, are separated, and separate electric currents are supplied to these elements. The voltages generated in the elements are added together in an adder circuit, and a resultant voltage is output from the adder circuit (see, for example, the specification of Japanese Patent No. 2778421 (page 3 and FIG. 1)). According to this loop filter, the electric current supplied to the capacitive element is smaller than that supplied to the resistive element, whereby the filter characteristics equivalent to those of a conventional filter are maintained, and the size of the capacitive element is relatively decreased.
The second countermeasure example is a low-pass filter disclosed in a patent application in which the present inventors are concerned (WO 03/098807 A1). In this low-pass filter, a filtering process of an input signal is performed by first filter means, and a filtering process of a second electric current generated based on first electric current flowing through the first filter means is performed by second filter means. Further, the first and second voltages generated in the first and second filter means, respectively, are added together by adder means and a resultant voltage is output from the low-pass filter. In this circuit, the second electric current is generated so as to be smaller than the first electric current, whereby the size of a capacitive element of the second filter means is relatively decreased while the filter characteristics equivalent to those of a conventional low-pass filter are maintained.
In the above first and second examples, the objective of reducing the size of the capacitive element is achieved, but on the other hand, collateral problems emerge. For example, in the first example, it is necessary to provide an adder circuit even when a passive loop filter is constructed, and accordingly, the circuit area and circuit complexity are increased. The second example is originally directed to an active loop filter and therefore basically includes an operational amplifier. Therefore, the second example does not require an additional addition means that is required in the first example. A problem in the second example is that the resistance value of a resistive element in electric current generation means which generates the second electric current is increased although the second electric current is decreased and the size of the capacitive element in the second filter means is decreased. The increase of the resistance value undesirably causes a deterioration in the noise characteristics because the resistor generates thermal noise.